The present invention relates generally to methods of removing interference distortion such as multipath distortion contained in signals obtained by demodulating frequency-modulated signals. More particularly, the invention relates to a method wherein a distortion component closely approximating an interference distortion contained in a signal obtained by demodulating a frequency-modulated signal is formed and used to cancel and remove the interference distortion.
In general, when a plurality of frequency-modulated signals (hereinafter referred to as FM signals) interfere with each other, interference distortion is produced. This phenomenon occurs in cases such as that, for example, wherein FM signals which have passed through a plurality of propagation paths (multipaths) interfere with each other, and this interference distortion is called multipath distortion. For example, in the case where an FM broadcast wave is directly received at the antenna of an FM receiver and, at the same time, is received after being reflected or diffracted by objects such as buildings, the FM signals thus received after passing through multipaths in this manner interfere with each other and give rise to interference distortion in the demodulated signal.
In order to reduce the generation of multipath distortion of this character, it has been a practice in known FM receivers to use antennas of sharp directional characteristic by which only directly arriving FM signals can be readily received, while FM signals arriving after being reflected or diffracted are not easily received. However, the occurrence of multipath distortion cannot be fully reduced by merely relying on only the directional characteristic of the antenna.
Accordingly, a method wherein a signal approximating the interference distortion component is formed by an electrical circuit within the FM receiver and used to cancel the distortion component in the demodulated signal has been tried. In this known method, however, the degree of approximation of the signal caused to approximate the distortion for the purpose of canceling the distortion component is very rough. For this reason, the signal components of the signal for distortion cancellation and the actual distortion component, strictly speaking, are considerably far apart, whereby distortion cancellation cannot be satisfactorily carried out, and, even after the distortion cancellation operation, a substantial distortion component remains.
In this connection, mutual interference of FM signals is not limited to only FM broadcasting but occurs in other instances such as the case of recording and reproducing of an FM signal on and from a recording medium and the case where there is mismatching between a cable for transmitting an FM signal and the related input and output equipments.